1. Technical Field
The present disclosure relates to technology for the measurement of characteristics whereby voltage is applied to the gate of an FET and the drain current that flows to the FET as a result is measured, and in particular, to a measurement apparatus for FET characteristics having both a function for DC (direct current) measurement whereby DC signals are applied to a gate and a function for AC (alternating current) measurement whereby AC signals are applied to a gate.
2. Prior Art
Microstructuring of the IC (semiconductor integrated circuit) has led to new means being introduced to technology for the assessment of FET characteristics. When measuring the IV characteristic (voltage-current characteristic) of an advanced MOSFET that uses SOI (Silicon-On-Insulator), strained silicon, high-k (high permittivity) materials, and the like, the heat generated by the FET itself makes it impossible to obtain very precise IV characteristic measurement results by conventional measurement methods in which DC voltage is applied to a gate. Therefore, a measurement method is known whereby measurement results that are not affected by the generation of heat are obtained by applying pulses with a short time width to the gate, as in IEEE Electron Device Letters, Vol. 16, No. 4, April 1995, K. A. Jenkins and J. Y-C. Sun, pp. 145-147.
Moreover, a measurement apparatus that operates by the above-mentioned new measurement method (referred to as pulse IV measurement hereafter) is cited on page 16 of IEEE ICMTS (International Conference on Microelectronic Test Structure) Textbook of Tutorial Short Course, Session 4: Characterizing Transient Device Behavior Using Pulse I-V Technique, Mar. 6, 2006, Y. Zhao, and it is reported that this apparatus is capable of DC measurement without changing the connection. FIG. 4 of the present Specification shows the measurement apparatus on the page 16 of the cited text book as conventional measurement system 700 for FET characteristics.
The hypothetical operation of conventional measurement apparatus 700 for FET characteristics will be described while referring to FIG. 4. The output of a pulse generator (PG) 706 is connected to the gate terminal of an FET 712 of a device under test (DUT); a DC measuring unit 702 is further connected between these via a bias-T 710; and an oscilloscope 704 is further connected between these via a pick-off T 708.
Oscilloscope 704 is connected to the drain terminal of DUT 712 via a bias-T 714, and DC measuring device 702 is further connected to the drain terminal via a bias-T 714. DC measuring device 702 also functions as a controller and controls oscilloscope 704 and pulse generator 706. The specific structure of pick-off T 708 is not described in detail in the aforementioned text book and therefore will be treated as a type of black box means for transmission of signals to the oscilloscope in the present Specification.
It is estimated that when characteristics are measured by applying a pulse to the gate terminal of DUT 712, a pulse waveform is output from pulse generator 706 as applied voltage Vg, this pulse is monitored by oscilloscope 704, a bias voltage from DC measuring unit 702 is applied to the drain terminal of DUT 712 via bias-T 714, and fluctuations in voltage Vd that are manifested at drain terminal of DUT 712 as a result of this pulse are monitored by oscilloscope 704 via bias-T 714. It is estimated that in case that a predetermined baseline voltage of the pulse is required for the pulse to the base terminal of DUT 712, a bias voltage is added from DC measuring unit 702 via bias-T 710, because the DC component of the output from pulse generator 706 is blocked by the capacitor of bias-T 710, as will be discussed later.
When DC measurement is performed, DC voltage is applied from DC measuring unit 702 to the base terminal of DUT 712 via bias-T 710 and the voltage at the drain terminal of DUT 712 is measured by DC measuring unit 702 via bias-T 714.
The bias-T will now be described while referring to FIG. 5. FIG. 5 shows the structure of a typical bias-T 800. A DC port 802 and an AC+DC port 806 are connected via an inductor, and an AC port 804 and an AC+DC port 806 are connected via a capacitor, in essence, a condenser. Consequently, based on the purpose for which it is operated, it is estimated that DC measuring device 702 is connected to the DC port of bias-T 710 in FIG. 4, and pulse generator 706 is connected to the AC port, while DC measuring unit 702 is connected to the DC port of bias-T 714 and oscilloscope 704 is connected to the AC port.
The following problems are encountered with the use of the above-mentioned measurement apparatus 700 for FET characteristics.
First, a long delay becomes necessary when switching from pulse IV measurement to DC measurement. The reason for this is that the capacitors housed inside the two bias-Ts are used for preventing leakage of direct current signals to the AC ports, and a capacitor with a relatively large value of several μF is usually used. Therefore, transient current for charging the capacity of the capacitor of bias-T 710 flows for a relatively long time beginning immediately after the DC voltage has been applied from DC measuring device 702 to the gate terminal. Consequently, it must not be measured during this period in order to obtain an accurate measurement. On the other hand, it is also necessary to wait until the transient current for the capacitor of bias-T 714 connected to the drain terminal has subsided in order to obtain an accurate measurement.
In this case, it goes without saying that the measurement will be inaccurate if DC measurement is performed without setting a sufficient delay time.
Next, the error in the measurement of the pulse waveform that will be applied to the gate terminal is large during pulse IV measurement, and there is therefore the possibility that there will be a large error in the measurement of the IV characteristic. By means of conventional measurement apparatus 700 for FET characteristics, pick-off T 708 for monitoring the pulse voltage is disposed in front of bias-T 710. This is apparently done in order to avoid a situation where the correct pulse voltage measurement cannot be obtained due to the current flowing from pick-off T 708 to oscilloscope 704 if pick-off T 708 is inserted behind bias-T 710. Nevertheless, because of its structure, it is difficult to obtain a bias-T having a sufficiently wide band pass and flat characteristics; therefore, the waveform of the pulse becomes distorted. Consequently, the pulse voltage waveform that is measured by oscilloscope 704 and the pulse voltage waveform actually applied to the gate terminal of DUT 712 are different, and this increases the possibility of an error in the measurement of the IV characteristic. As an example describing this phenomenon, FIG. 6 shows waveform P at the AC port (solid line) and waveform Q at the AC+DC port (broken line) when a pulse of 100 nsec, which is approximately the condition used in pulse IV measurement, has been applied to a commercial pulse bias-T having a usable frequency band of 12 kHz to 15 GHz. Judging from FIG. 6, the slope of waveform Q is long from the rise in the pulse until it gets settled, and there is a perception of undulation once it does get settled, but this type of distortion is not obvious when waveform P is measured.
When the pulse parameters (period, width, rise time, fall time) are changed, it becomes necessary to switch the both of the two bias-Ts with conventional measurement apparatus 700 for FET characteristics, and this increases operating time and material cost. In essence, the bias-T acts as a band-pass filter; therefore, it is necessary to carefully select the bias-T in accordance with the pulse parameters when measurement is performed using pulses. As a result, it is necessary to switch between two bias-Ts with changes in the pulse parameters that are used.